(a) Field of the Invention
The present invention relates to a hydrogen supply apparatus of a fuel cell stack. More particularly, the present invention relates to a hydrogen supply apparatus of a fuel cell stack that temporarily supplies hydrogen to a cathode to improve performance and durability of a fuel cell stack.
(b) Description of the Related Art
As is generally known, a fuel cell system is a type of power generation system that directly converts chemical energy of a fuel to electrical energy. This electrical energy can then be used to provide power to vehicles, electronic devices and any other device that operates off of electricity.
Typically, a fuel cell system includes a fuel cell stack that generates electrical energy, a fuel supply apparatus that supplies fuel (e.g., hydrogen) to the fuel cell stack, an air supply apparatus that supplies oxygen (i.e. air) to the fuel cell stack, and a heat and water management apparatus that disperses reaction heat from the fuel cell stack to the outside and controls operating temperatures of the fuel cell stack.
Fuel cell systems generate electrical power via an electrochemical reaction, and exhaust heat and water that are by-products of the electrochemical reaction.
When a fuel cell stack is applied to a fuel cell vehicle, because of the high power output required, a stack is assembled in which unit cells are continuously and consecutively stacked one on top of the other. A membrane electrode assembly (MEA) is provided to the innermost part of each of the unit cells. This membrane electrode assembly typically includes a polymer electrolyte membrane for transferring protons and a catalyst layer provided on both sides of the polymer electrolyte membrane respectively. Additionally, the catalyst layer includes a cathode and an anode.
A gas diffusion layer (GDL) is also provided on both sides of the membrane electrode assembly respectively along with a separating plate (or separator) within which a flow field is formed. The separating place is typically disposed to abut an outer side of the gas diffusion layer (i.e., the side not in contact with the MEA) This GDL and separator combination are responsible for supplying fuel and air to the cathode and the anode and discharging water generated by the chemical reaction.
Hydrogen and oxygen are ionized by the chemical reaction of each catalyst layer, thus generating an oxidation reaction that generates electrons within a hydrogen portion of the cell and a reduction reaction that generates water within an oxygen portion of the cell. Generally, an electrode catalyst applied to the fuel cell includes a catalyst support made of a carbon material and a catalyst including a platinum catalyst and a co-catalyst (for example, Ru, Co, and Cu).
That is, the hydrogen is supplied to the anode, and the oxygen (air) is supplied to the cathode. Therefore, the hydrogen supplied to the anode is divided into protons (H+) and electrons (e−) by a catalyst of an electrode layer provided at both sides of an electrolyte layer. Only the protons (H+) are selectively transferred to the cathode through the electrolyte layer of the positive ion exchange layer. Simultaneously, the electrons (e−) are transferred to the cathode through the gas diffusion layer and the separating plate.
In the cathode, the protons supplied through the electrolyte layer and the electrons supplied through the separating plate have a chemical reaction with oxygen in the air supplied to the cathode by an air supplying apparatus and generate water. Movement of the protons generates a current, and heat is generated in a water generating reaction.
A starting and stopping operation frequently occurs while driving the fuel cell stack. However, when a high voltage or a reverse voltage is generated in the fuel cell stack, performance of the fuel cell is deteriorated
In the case of a fuel cell installed in a vehicle, a load on the fuel cells rapidly changes. Particularly, when the vehicle starts from an idle state, the load is rapidly changed as the vehicle accelerates. When the load changes in this manner during operation, the fuel cell stack has low humidity and the vehicle is in an idle state, a cell voltage rapidly drops and is restored to the initial state. Since mobility of H+ ions is degraded in a dry state, the entire reaction speed is decreased, causing a phenomenon to occur.
Additionally, the catalyst of the fuel cell stack is degraded due to continuous usage of the fuel cell. This results in a deteriorated output performance of the fuel cell.
When the fuel cell is started at low temperatures (i.e., below freezing), such as in winter, heat generated by the electrochemical reaction of hydrogen and oxygen is used to start the fuel cell. However, since an amount of heat and thermal capacity of peripheral components is quite low, the temperature of the fuel cell stack increases slowly when the fuel cell is started at low temperatures. Finally, noise is generated when hydrogen of the fuel cell stack is purged. This noise causes an uneasy feeling to a driver and thus should be avoided if possible.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.